This invention concerns an automatic dimmer which can automatically control the quantity of light received to suit the image pickup in an endoscope which uses a solid pickup element as the image pickup means.
Recently the endoscope which uses a solid pickup element and makes it possible to display the picture of a subject on a display device such as CRT is being realized.
In comparison with those which form the optical image on the image guide fibers, the electronic endoscope which uses the above solid pickup element can more easily record the image and has an advantage that it can be made smaller and smaller, as the high integrating technology advances.
But when the solid pickup element is used and if the quantity of light incident upon the light receiving elements on the image pickup face is too big, excess charge leaks in the periphery causing smearing and blooming and making it impossible to reproduce a true image of that portion and the image cannot be picked up until the normal state is recovered.
There are some devices which can mechanically control the strength of the illuminating light on the light source side to prevent the blooming phenomenon, but it is almost impossible to achieve a proper intensity of illumination in a short time because the control must be made in accordance with the distance to the subject when they are used in a human body and also the distance to the inside wall of the body changes in a complicated way when the end side of the inserting member is bent.
And when they are used in a human body, the reflecting strength increases when the affected part is wet with body fluids, and the proper intensity of illumination changes depending on the state of the affected part. To the contrary, if the intensity of illumination is too low, the affected part cannot be picked up clearly and good diagnosis cannot be achieved.